The HIV Preclinical Vaccine Development Master Agreement Program was initiated to enable the NIAID to provide resources to accelerate the development of promising vaccine approaches. Under this program, a series of vaccine studies have been initiated to evaluate candidate and prototype AIDS vaccines in nonhuman primates. These studies are designed to obtain more information about the efficacy and optimum presentation of candidate HIV vaccines that are in human Phase I clinical trials or proposed for such trials as well as prototype SIV vaccines of the same type as these HIV vaccines. Other vaccine studies will evaluate prototype HIV or SIV vaccines that offer novel presentation systems, may be capable of eliciting broadly cross-reactive immune responses, or have shown promising preliminary results. NIAID has initiated collaborative arrangements with researchers who have agreed to provide HIV and SIV vaccines for immunization of nonhuman primates. In all of the vaccine studies, the nonhuman primates will be challenged after immunization to determine if protection from infection or from disease can be obtained. The contractors in Category B of the Master Agreement Program will provide, house and care for, immunize, and challenge the nonhuman primates for the various vaccine studies. The contractors will obtain sera, cells, and other appropriate samples from the animals in the studies, and will ship the samples, as appropriate, to contractors for evaluation of humoral immune responses, including neutralizing antibodies, for evaluation of cellular immune responses, and for virological assessment after challenge. The category B contractors will maintain a data base for each study they are conducting in order to follow the immunization protocol, virus challenge, and all the assay results for each monkey in the study.